The typical width of a weld for a directionally or single crystal article build-up is generally limited to the width of a single weld bead (see FIG. 1) which is generally up to 1.9 mm wide. However, many components often have surfaces which require a larger width build-up than a single weld bead. Using parallel and overlapped weld beads to enlarge the build-up width is common in laser powder welding repairs. However, due to the inhomogeneous solidification rate between the beads, a significant amount of micro-grains are created (see FIG. 2) which is generally unsuitable for directionally or single crystal articles, thus limiting the welding process to a single weld bead width.
Nickel based single crystal or directionally solidified (DS) superalloys (hereinafter jointly referred to as single crystal superalloys) with a high volume fraction of gamma prime precipitates are susceptible to strain age cracking resulted from welding. Therefore many weld repair processes are carried out using conventional solid solution strengthened welding alloys such as Inconel 625 or an oxidation resistant high strength Co based weld fillers. However, many weldable Ni fillers such as IN-625 due to its low Al and Ti are softer than most of the nickel based single crystal superalloys, which is not desirable for component tip or edge repairs that require considerable strength. Further, the large thermal expansion mismatch between the high strength Co or Ni fillers and the nickel superalloy substrate could produce high thermo-mechanically induced damage and low TMF (thermal mechanical fatigue) life during service. To extend the repaired component life, ideally, the weld metal used should have either the same or very similar composition as the base metal so that the thermal expansion and creep properties of the weld will closely match the base metal. This is particularly attractive for nickel based single crystal superalloy components. However, it is very difficult to produce a crack free weld with any nickel based single crystal alloy filler as the presence of high Al and Ti contents in the filler results in substantial shrinkage strains from gamma prime precipitation.
The prior art discloses two major approaches to eliminating the cracks in the nickel base superalloy weld build-up: 1) laser powder weld build-up while heating the component substrate (see U.S. Pat. No. 5,106,010, U.S. Pat. No. 6,037,563, EP 0861927 and U.S. Pat. No. 6,024,792); and 2) using low energy laser beams to re-heat each deposited layer (U.S. Pat. No. 5,900,170, U.S. Pat. No. 5,914,059 and U.S. Pat. No. 6,103,402). While heating the components during laser powder welding is effective, it is an expensive process, can cause distortion of the component and has the potential of affecting the microstructure of the single crystal alloy and for DS alloys there can be incipient melting at the grain boundaries.